In my U.S. Pat. No. 4,057,699 there is disclosed a control device for maintaining a predetermined pressure in pressurized conductors provided with temperature condensation in which SF6 is used in the pressurized chamber. In that application, there are opposed bellows responsive, respectively, to pressure in the pressurized chamber and switch means operable thereby to supply sufficient pressure to the pressurized chamber to compensate for a loss of pressure therein, and to a rise in temperature in the pressurized chamber to predispose the pressure-responsive bellows for a change in temperature in the pressurized chamber. The device of the invention operates in the same way using nitrogen in the pressurized chamber, employing diaphragms in place of a bellows structure such that it can be operated at both low and high pressure ranges and its construction and operation is vastly simplified and it sensitivity improved, thus reducing manufacturing costs, repair and adjustment.